Ancient and Modern History

An ancestral Mount Pinatubo began to grow at least 1 million years ago and produced a large but unremarkable stratovolcano. Eruptions of the modern Pinatubo began with an unusually large explosive event slightly more than 35,000 years ago and have occurred episodically (Newhall and others). That of >35,000 years ago was roughly ten times as large as that of 1991, and early reposes of thousands of years appear to be much longer than the most recent repose of 500 years. Burial and erosion of deposits from older, frequent, small-volume eruptions might create an artifact of change from big eruptions and long reposes to smaller eruptions and shorter reposes, but there is also a clear decrease in the size of Pinatubo's largest eruptions per eruptive episode. Except for slight variability in SiO2 contents of the main dacitic products from one eruptive period to the next (SiO
2 is highest in the products of the oldest, largest eruption of the modern Pinatubo), eruptive products have been remarkably uniform through time. This uniformity in eruptive products suggests that they are drawn from a single, large volume reservoir.

Exploratory geothermal drilling at Mount Pinatubo in 1988-90, fortuitous for science though not for the drillers, revealed high-temperature (261-336°C), highly acidic fluids and low permeability (Delfin and others). The prospect was abandoned for economic reasons; no one foresaw that most of those fluids would soon be erupted. Were it not for the requisite cementing those wells might have provided an unparalleled window into the temperature, pressure, and chemistry of a hydrothermal system as the volcano drew closer to its eruption! Not all was lost: highly acidic, high-Cl fluids at high elevations on a volcano are now understood to be a clear magmatic signature in geothermal fluids.

Stacked pyroclastic-flow and lahar deposits outside Clark Air Base, testimony to a long history of explosive eruptions.